fix condition for stdin and allow piping to stdout

@kevva this shld fix it i guess ? thanks for the input related to #382 
The *help* test is failing due to this change.

* Removing `argv.o === undefined` is OK.
* Removing `!argv.stdout` fails the test.
* Keeping `!argv.stdout` and adding `process.stdin.isTTY` throws: "Error: timeout of 2000ms exceeded".